The present invention relates to a fuel cell and more particularly to a fuel cell including a solid polymer electrolyte for use in portable power sources, electric vehicle power sources, domestic cogeneration systems and the like.
A fuel cell using a solid polymer electrolyte causes an electrochemical reaction between a fuel gas, such as hydrogen, and an oxidant gas, such as oxygen in air, to simultaneously generate electric power and heat. Typically, such a fuel cell includes a polymer electrolyte membrane for selectively transporting ions and a pair of electrodes arranged to sandwich the membrane. Each of the electrodes includes a catalyst layer mainly composed of carbon powder carrying a platinum metal catalyst thereon and a gas diffusion layer having both gas permeability and electron conductivity formed on the surface of the catalyst layer.
In order to prevent the supplied fuel gas and oxidant gas from leaking out of the fuel cell or the two gases from mixing, a gas seal or a gasket is arranged to sandwich the polymer electrolyte membrane on the periphery of the electrodes. The gas seal or the gasket is integrated in advance with the electrodes and the membrane. This assembly is known as MEA (membrane-electrode assembly). Next to the MEA, a conductive separator plate is arranged to mechanically secure the MEA and electrically connect the MEA with an adjacent MEA in series. In part of the separator plate contacting the MEA, a gas flow channel is formed to supply a reaction gas or exhaust generated gas or excessive gas to and from the electrode surface. The gas flow channel may be provided independently from the separator, but typically, a groove is formed on the surface of the separator plate which functions as the gas flow channel.
To supply a reaction gas to the grooves of the separator plates in a fuel cell, a pipe for supplying the gas is diverged into branches in a number corresponding to the number of the separators used. Therefore, a jig for connecting the branches to the grooves, respectively, is required. This jig is called a manifold. In particular, a manifold connecting the pipe and the grooves directly as mentioned above is called an external manifold. Another manifold of a simpler structure is called an internal manifold. The internal manifold can consist of a hole penetrating the separators. Inlets of the gas flow channels are formed to communicate with the hole, from which the reaction gas is directly supplied to the gas flow channels.
The hole should have a cross sectional area that is larger than the total cross sectional area of the gas flow channels of the combined separator plates. If the hole area is too small, a pressure loss in the manifold results and increases during the gas supply, thereby increasing the workload required for the gas supply which can be satisfied by using a blower or the like. As a result, the overall system may decrease in efficiency. In order to avoid such a problem and the unwanted pressure loss in the manifold, a larger area is required for the hole.
A fuel cell generates heat during operation. Therefore, it is preferable to cool the fuel cell to keep it within a suitable temperature condition. In general, a cooling section through which a cooling medium is passed is inserted between the separators in every one to three cells. In most cases, the cooling section is provided by forming a cooling medium flow channel on the rear surface of the separator plate. To form a common cell stack, the MEAs and the separators are can be arranged to form a stack of 10-200 cells. Then, an end plate is arranged on each end of the cell stack with an intervening current collector plate and an insulating plate. The cell stack and plates can be secured with fastening rods at the both ends.
As conventional fuel cells, Japanese Laid-Open Patent Publication No. 2002-343400 proposes a fuel cell in which a gas supply or exhaust pipe is arranged so that the bottom of the pipe's inner wall is positioned at the same level as or lower than the bottom of the manifold penetrating the separator plates. Further, Japanese Laid-Open Patent Publication No. 2003-223922 proposes a fuel cell in which an outlet (exhaust) manifold is vertically elongated so that the manifold communicates with an outlet of the gas flow channel at an upper part thereof and connected with a gas exhaust pipe at a lower part thereof.
Typical polymer electrolyte materials include polymers having perfluoro sulfonic acid and perfluoro sulfonate groups. In membrane form, this class of materials develop ion conductivity when they contain moisture and, thus, the fuel gas and oxidant gas are usually humidified before supplying the gases to the cell. However, if the supplied gas is humidified to have a dew point higher than the operation temperature of the fuel cell, condensation occurs in the gas flow channel or inside the electrode and, in extreme circumstances, accumulates water in the fuel cell. Additional water can accumulate because the reaction on the cathode side generates water. As a result, the performance of the fuel cell becomes unstable or deteriorated. Such a phenomenon caused by too much moisture, in which deterioration in fuel cell performance and unstable operation occur, is called flooding. If this phenomenon occurs on the anode side, a shortage of the fuel gas occurs, which is fatal to the fuel cell. More specifically, if the fuel cell is forced into operation in a state where the fuel gas is lacking, carbon associated with the anode catalyst reacts with water in the atmosphere to generate electrons and protons. Accordingly, carbon is leached from the catalyst layer, destroying the catalyst layer of the anode. Therefore, flooding on the anode side is of significant importance.
If the fuel cell is used for a power generation system, procedures including the humidification of the supply gas need to be systematized. For the purposes of obtaining a simple system and improving efficiency thereof, it is preferable to humidify the supply gas to have a dew point as low as possible. Therefore, in general, the supply gas is humidified to have a dew point slightly lower than the fuel cell temperature so as to avoid flooding, improve the system efficiency and simplify the system.
However, in order to achieve higher performance of the fuel cell, the polymer electrolyte membrane needs to be improved in ion conductivity. For that purpose, it is preferable to humidify the supply gas to have almost 100% relative humidity or higher. In view of durability of the polymer electrolyte membrane, it is also preferable to supply the gas with high humidity.
If the gas is humidified to have the relative humidity of almost 100%, the supplied gas causes condensation with a high possibility in the upstream of the fuel cell stack. The condensed water is then supplied to the stack in a mist state. If the separator plate surface is arranged parallel to the gravity direction and the inlet manifold for the gas supply is provided in an upper part of the separator plate in the gravity direction, the mist flows locally into cells which are relatively close to the gas supply pipe. As a result, flooding occurs in these cells, deteriorating their performance. If the inlet manifold is not provided in the upper part of the separator plate, the mist remains in the inlet manifold, making the gas supply unstable. Further, the amount of gas supply may differ among the cells.
In the fuel cell disclosed by Japanese Laid-Open Patent Publication No. 2002-343400, the gas supply or exhaust pipe is misaligned with the projected surface of the manifold. This may possibly cause a wasteful region that does not contribute to the power generation in the separator plate surface. On the other hand, Japanese Laid-Open Patent Publication No. 2003-223922 merely specifies the arrangement of the outlet manifold. Therefore, a phenomenon ascribable to an inlet manifold may possibly occur to make the operations of the fuel cell unstable.
To address the above problems, the present invention provides a fuel cell which allows a substantially uniform gas supply to the unit cells in the cell stack. More specifically, the present invention provides a compact, highly reliable solid polymer fuel cell, substantially free from the adverse retention of condensed water in a manifold and capable of supplying a gas uniformly and stably without deteriorating or destabilizing the fuel cell performance.